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Alkenes: Chemical Properties

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Alkenes: Chemical Properties - Lesson Summary

Alkenes are unsaturated hydrocarbons consisting of a sigma bond and a pi bond. The typical reactions of alkenes involve the breaking of the weak pi bond to form two sigma bonds. Such reactions are called addition reactions.

These addition reactions are usually electrophillic in nature as the pi electrons of carbon-carbon double bond are available to any species seeking electrons.

Addition reactions:
  • Addition of dihydrogen
  • Addition of halogens
  • Addition of halogen acid
  • Addition of sulphuric acid
  • Addition of water
  • Oxidation
  • Ozonolysis
  • Polymerization

Addition of dihydrogen to alkene:
Alkenes react with dihydrogen in the presence of catalysts like finely divided nickel, palladium or platinum to form alkanes and these are cis –addition reactions.



Ex: Ethene in the presence of finely divided nickel, palladium or platinum yields ethane.

Addition of halogens:
Alkenes react with halogens such as chlorine and bromine to form vicinal di-haloalkanes. The alkenes and the halogens are mixed together in an inert solvent like carbon tetrachloride. Ethene reacts with bromine to give Trans 1, 2 di-bromoethane. During the addition reaction between ethene and bromine, the reddish brown colour of bromine gets decolorized due to the formation of di bromoethane. Hence, this reaction is used as a test for unsaturation.


 
Addition of halogen acid:
Alkenes react with halogen acids to form haloalkanes. Alkenes towards hydrogen iodide are more reactive than hydrogen bromide which inturn more reactive than hydrogen chloride. In the case of unsymmetrical alkene, two products are formed.



Ex: Propene reacts with hydrogen bromide to form 2-Bromopropane and 1-Bromopropane among which 2-Bromopropane is the major product in accordance with Markownikov”s rule. The addition of hydrogen bromide to unsymmetrical alkenes like propene in the presence of benzoyl peroxide is on the contrary to Markownikov”s rule and is known as anti-Markownikov”s addition or peroxide effect or Kharash effect and this goes through free radical mechanism.

Alkenes react with cold concentrated sulphuric acid in accordance with Markovnikoff's rule to form alkyl hydrogen sulphates.
Ex: Ethene and propene react with cold concentrated sulphuric acid and form ethyl hydrogen sulphate and propyl hydrogen sulphate respectively.

Oxidation reactions of alkenes:
Alkenes on reacting with cold, dilute, aqueous solution of potassium permanganate which is also called Bayer’s reagent produce vicinal glycols, that is 1, 2-Diols.

Ex: Ethene on reaction with Bayer’s reagent forms ethane 1, 2-diol or ethylene glycol. As the purple colour of potassium permanganate decolorizes with the formation of glycol, this reaction is used as a test for unsaturation. Addition of ozone to an alkene is also called Ozonolysis.

Alkenes react with ozone to form ozonide, which on further hydrolysis in the presence of zinc form either Aldehydes or Ketones or both. Ex: When ozone molecule is added across the double bond in ethene to give ethene ozonide. This on hydrolysis forms formaldehyde or methanal.

Polymerization:
Alkenes undergo addition polymerization reaction when heated under pressure, in the presence of suitable catalysts. In this, a large number of molecules of the same species join together to form a giant molecule, called a polymer. The simple compounds which form the polymers are called monomers.


Ex: Ethene when heated to 1000 °C less than 1000 atmospheric pressure undergoes polymerization to form polythene.

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